Density-functional study of the adsorption of benzene on the (1 1 1), (1 0 0) and (1 1 0) surfaces of nickel

The adsorption of benzene on all three low-index surfaces of nickel has been studied using gradient-corrected density-functional calculations. Our technique is based on ultrasoft pseudopotentials, residuum minimization techniques for the calculation of the electronic ground-state and of the Hellmann–Feynman forces and stresses, and on a conjugate-gradient technique for the optimization of the atomic structure. The surfaces have been modelled by periodically repeated slabs with up to six-layer slabs, allowing for the relaxation of the uppermost layer. For Ni(1 0 0) and Ni(1 1 0) surfaces an adsorption with the centre of the aromatic ring placed above the hollow position has been identified to be energetically most favourable, whereas for the Ni(1 1 1) surface adsorption in the bridge position results in slightly higher binding energies. Adsorption-induced distortions of the molecular geometry are found to be modest in all cases: the C–C bond distances are slightly elongated, but the differences in the bond lengths never exceed 0.03 Å. The aromatic ring remains flat, but the H atoms are tilted away from the surface of the substrate. We also present a detailed analysis of electronic structure of the adsorbate/substrate complex and of the charge flow induced by the adsorption. Our results are discussed in relation to recent experiments and other theoretical studies.